Question: Which of these numbers is prime? ${35,\ 51,\ 59,\ 90,\ 91}$
A prime number has exactly two factors: 1 and itself. The factors of 35 are 1, 5, 7, and 35. The factors of 51 are 1, 3, 17, and 51. The factors of 59 are 1 and 59. The factors of 90 are 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, and 90. The factors of 91 are 1, 7, 13, and 91. Thus, 59 is a prime number.